Plasma has long been employed to process substrates (e. g., wafers or flat panels) to form electronic products (e.g., integrated circuits or flat panel displays). In plasma processing, a process gas may be injected into a chamber and energized to form a plasma to either deposit a layer on the substrate or to sputter or etch the substrate. In some processes, particularly those involving deep etching of the silicon layer, there exist various etch techniques that alternate etching and deposition sub-steps in order to perform the etch more anisotropically (e.g., forming sidewalls of the trenches or holes more vertically).
One of these techniques is known as the Rapid Alternating Process (RAP) process, which alternates etching and deposition cycles during etching. In the RAP process, different process parameters are utilized for the alternating deposition and etching cycles. These process parameters may include, for example, different chemistries, pressures, bias voltages, TCP (inductive coil) voltages, and the like. The alternating cycles are typically executed such that a deposition cycle would be followed by an etch cycle and then a deposition cycle and so on. The overall composite etch step may involve hundreds or even thousands of these alternating cycles.
When the cycles change from deposition to etching (or vice versa), there exists a transition period when transients in the chamber parameters make it challenging to perform power matching. As is well known in the art, plasma may be generated in inductive chambers using an antenna (such as an inductive coil), an RF generator and a match network. The match network is employed to ensure that the power delivered is matched with the load in order to minimize the reflected power and thereby maximizing power delivery to the load.
Generally speaking, a mechanical RF match tends to employ tunable capacitors whose capacitor values may be changed in order to accomplish the power matching. This mode of operation is referred to herein as the mechanical tuning mode. During the transition period between cycles, the mechanical RF match that is typically employed to perform power matching may have difficulties handling the rapidly fluctuating transient parameters. Furthermore, since a given RAP etch step may involve hundreds or even thousands of alternating cycles, the mechanical tuning mode (i.e., power matching by changing the settings of the tunable capacitors) tends to wear out the tunable capacitors fairly quickly. Due to these issues, frequency tuning has been proposed for performing power matching in plasma processing systems that employ mechanical RF match networks.
In frequency tuning, the capacitors in the mechanical RF match network are fixed at some values and the RF power supply varies the frequencies of the RF signals in order to match the power delivered to the load. Since the power delivered typically involves at least two components (e.g., real and imaginary), two tunable capacitors are typically employed.
To optimize power matching using frequency tuning, however, each of the tunable capacitors needs to be fixed at some value that is optimized for the specific recipe employed and/or the specific conditions of a given chamber. If the capacitors of the mechanical RF match are set at their optimal values for the recipe being employed and/or the specifics of plasma processing system involved, power matching via frequency tuning can be made more efficient.
In the prior art, the process of determining the capacitor values for fixing or setting the tunable capacitors in a mechanical RF match prior to performing frequency tuning tends to involve a manual and laborious trial-and-error approach. For example, multiple sample substrates may be processed multiple times in a given chamber to build a matrix. The optimal capacitor values for the tunable capacitors may be extracted from the matrix once processing of the multiple sample substrates is completed. After the optimal capacitor values are extracted or determined, these capacitor values can be employed to set the positions of the tunable capacitors, thereby setting the values of the tunable capacitors at the values that are determined to be optimal by the prior art process.
However, the manual, laborious, and time-consuming nature of the prior art capacitor value determination process negatively impacts the system substrate throughput. Further, the process tends to be error-prone due to the involvement of a human operator. Because of this, the values of the tunable capacitors are typically determined only once in advance and then employed for running multiple substrates since it is fairly costly and time consuming to perform the prior art capacitor value determination process
It has been found, however, that chamber conditions do not stay unchanged over time. Phenomena such as chamber drift (e.g., a situation in which the chamber conditions change from substrate to substrate due to, for example, polymer deposition) renders the tunable capacitor values that are found to be optimally for the first substrate in the batch non-optimal for the Nth substrate in the batch. Accordingly, the etch result for the first substrate would be different from the etch result for the Nth substrate, thereby impacting the repeatability of the etch process and affects the quality of the end product.
Furthermore, the manual, laborious, and time consuming approach of the prior art capacitor value determination process, which involves processing multiple sample substrates prior to extracting the optimal capacitor values, also makes it nearly impossible to determine capacitor values on-the-fly in the middle of a long ramped RAP step. In a ramped RAP step, one or more parameters may change as the RAP step is executed. To elaborate using an example, as the etch proceeds deeper and deeper into a layer on the substrate to create a deep trench and as the trench is being alternatively etched and deposited using alternating cycles of a RAP step, the optimal parameters for etching that exist at the beginning of the RAP step would be different from the optimal parameters for etching at the Nth cycle of the RAP step. To ensure a satisfactory etch, chamber parameters (such as pressure, bias power, gas mixtures, etc.) may be changed during a long ramped RAP step. The change in these chamber parameters requires the power matching to be updated, which involves updating the tunable capacitor value settings since the capacitors values that are set at the beginning of the ramped RAP step may no longer be optimal at the Nth cycle of the ramped RAP process.
Ideally, the tunable capacitor values would be ascertained just prior to changing the tunable capacitor value settings. As mentioned, a long ramped RAP step may require multiple capacitor value setting changes in order to adapt to condition changes. However, since the prior art process would require the cessation of the current RAP step, opening the chamber and running multiple sample substrates, and extracting the optimal tunable capacitor values from the sample substrate runs, it is impossible to duplicate conditions that exist at a given cycle in the middle of a RAP step to facilitate the discovery of the optimal tunable capacitor values at that given cycle in the middle of a RAP step.
In the view of the foregoing, there are desired improved, more efficient, and more automated process for discovering the optimal values for the tunable capacitors of a mechanical RF match network for a frequency tuning/power matching application in a plasma processing system.